Cavity resonator circuit



Sept. 5, 1944. f P. s. CARTER 3 3 CAVITY RESONATOR CIRCUIT Filed Jan. 4,-1941 5 Sheets-Sheet 1 F I ou/rpur 4 Fig.4

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- PHIL/P 5. CARTER X BY 3 ATTORNEY.

Sept. 5, 1944. P. s. CARTER CAVITY RESONATOR CIRCUIT Filed Jan. 4, 1941 s sheets-sheet 2 Fly. (1

E m Mm WM ER m INVENTOR. p/wigs. CARTER ATTORNEY.

Sept. 5, 1944. P, s CARTER CAVITY RESONATOR CIRCUIT 3 Sheets-Sheet 3 Filed Jan. 4, 1941 w m F INVENTOR. PHl-L/P 5. CARTER A TTORNE Y.

Patented Sept. 5, 1944 UNITED CAVITY RESONATOR cmcUrr Philip S. Carter, Port Jefferson, N. Y., assignorto- Radio Corporation of America, a corporation of Delaware Application January 4, 1941, Serial No. 373,072

22 Claims.

This invention relates to coupling circuits for passing a band of frequencies, and particularly to such circuits employing cavity resonators. The term cavity resonator is intended to include any high frequency electrical resonator comprising a closed electrically conducting surface enclosing a hollow space, and wherein the enclosure contains a periodically repeating electromagnetic field. The term coupling circuit" used herein is intended to include any circuit which selectively passes a bandof frequencies, such as for.

example an electrical wave filter, or a selective circuit, which might be used .between stages of a receiver or transmitter.

In the communication field, it is often.desirable to employ a four-terminal-band pass coupling circuit which has two or more natural-ire.

mental modes of oscillation to provide a band pass characteristic enables me to use cavity resonators of reduced size in at least two dimensions.

One of the objects of the present invention is to provide a cavity resonator coupling circuit so excited that there is caused to exist two or three fundamental frequencies of oscillation differing by a predetermined percentage of the mid frequency and which possesses a desired band pass quencies of oscillation differing by a small predetermined percentage of .the mid frequency.

Such four-terminal circuits may take the form of two or more coupled tuned circuits, one being connected to the input terminals and another to' the output terminals, or maytake the form of any suitable impedance network. It is known I that such circuits may be made to obtain a band pass characteristic when loaded with a resistance.

This resistance, which may constitute the useful load per se, serves to smooth out the multi-peak resonance of the four-terminal coupling circuit.

It has been found, however, that when using ultra high frequencies it is impractical toconstruct such circuits of coils and condensers.

In my copending application Serial No. 359,187, filed October 1, 1940, there are described several types of band pass cavity resonators wherein use is made only of modes of oscillation in which the electric field is entirely inone direction. When the electric field is entirely in onedirection, let us 'say vertical (by way of example) then the natural frequencies of oscillation will =-be-determined entirely by the dimensions of the base,

and there will exist only one fundamental natural frequency. In order to obtain two natural frequencies lying close together, in accordance with the teachings of my copending application,

employed, in accordance with the teachings of the invention, in order to provide a band pass characteristic, are only the fundamental modes of oscillation. thus differing from the modes of oscillation employed in my copending application, Serial No. 359,187, which are of a higher order than the fundamental. One advantage of the cal standpoint, is the simplest embodiment show 1 line TL terminating in .a dip le m in the interior present invention over that disclosed in my 00- pending application is that the use of the funda- 7 shown).

characteristic. Where the cavity resonator of the invention has three fundamental frequencies of oscillation, it is contemplated that one of the fundamental frequencies of oscillation corresponds to the mid frequency of the band pass.

The following is a description of the invention accompanied by drawings wherein:

Figs. ,1, 2,. 3 and 4 represent different cavity resonators. constructed in accordance with the principles of the present invention; I

Figs. la, 1b and lcare respectively plan, side and end elevations of'Fig. 1;

Fig. 5 is a curve given to aid in an understanding of the prin ples involved in connection with the resonator 0 Fig. 4;

Fig. 5a illustrates the electric field configure--- tion for a'mode of oscillation present in the structure of Fig. 4;

,Figs. 6a, 6b and 6cillustrate plan, side and end elevations respectively of another embodiment of the present invention; Figs. 7a, 7b and 7c illustrate plan, side and end elevations respectively of a further embodiment of the present invention and Figs. 8a, 8b and 8c illustrate plan, side and end elevations respectively of a still further embodiment of the present invention.

Fig. 1 illustrates a cavity resonator in accordance with the invention which, from a theoretiing how to make use of three types of wavesand their fundamental modes ofoscillation. This figure represents a rectangular prism whose sides are indicated by the dimensions a, b, c of different predetermined lengths, lying along the directions :c, 1/, 2 respectively. Atone corner of the prism there is shown an' input transmission of the prism. The axis of this dipole mis arranged to make equal angles of 54.7- with the three directions at, y, z in order to obtain equal excitation of all three types of waves. Transmission line TL, of course, is coupled to a suitable source of high frequency oscifiations (not At the diagonally opposite corner of the prism, Fig. 1, and in its interior is a second dipole n coupled to a transmission line TL form-. ing the output circuit. The axis of dipole n is p l to the axis f dipole m and hence also makes equal angles of 543 with the directions I 0:, y, and z. The cavity resonator of'Fig. 1 is thus .ing difi'erence in thexdciree o three modes of oscillation; It might be desirable,

' quenci'es, which I For such a resonator let xx, x, and X; indicatethe natural wavelengths corresponding to the three fundamental modes of oscillation, the sub letters indicating the direction of the electric field in'the standing wave. when the electric field is entirely vertical and in the z direction, the natural wavelength is determined entirely by the dimensions a, ,b of the base, and is given by the relation l 'Whenthe electric field is entirely horizontal and in the y direction, the natural frequency is determined entirely by the dimensions a and c, and is given by the relation 2ac 4m When the electric field is horizontal and in the a: direction, the natural frequency is determined by the dimensions c and b and given by the relation 5 A c .1/ "+c*' From the relations given above in connection with ix, i M, wethus are able to obtain any. three natural frequencies desired by suitably choosing the lengths of the sides.

Ifwthe dimensions-.0, b and c of Fig; 1 were all made to be equal, the resulting structure would be a cube, and the three natural modes of osci1la-.

, tion would coincide in frequency. The electromagnetic field within the resonator of the cube when fedin the manner shown in Fig. 1 would then be the equivalent of a superpositioning of the three types of waves above mentioned and the electric fieldiof the resultant oscillation would "15 be in the direction of the diagonal from one corner to the opposite corner of the cube; that is, the diagonal which is parallel to the axis of the dipolesm and n.

Although the dipoles m and n have been shown 5 in Fig. 1 as'being 'so arranged that they make equal angles, of 543 with the three directions a, v and z, it should be understood that the axis of these dipoles may make different angles with respect to the directions :c, y and z,-with a result excitation of the under some conditions, to tend to underei'cite.

the mid frequency of the three fundamentalfie.

mld frequency'might corresp nd no to the 'wave having itselectrlc field in the z direction. This can be-doneby; decreasing the angle of the dipole-axis with respect to the direction.

Where its desired toobtain aband pass-chara5 acteristic produced by the use of a circuit havin: two natural fundamental frequencies rather than three, a structure suchas shown in can be employed. Fig. 3 shows a rectangular prism having an input toriv line TL entering the middle of the 'left'hand side L and terminating in a dipole m. The axis of the dimm m is parallel to the zy plane and is arranged to make equal angles of with the directions z' and u. The output dipole n is parallel to dipole m and is located in the interior of the shown) coupled to the transmission line TL, tends to excite oscillations of equal amplitude in the resonator wherein the electrical field is either vertical in direction z or horizontal in the direc-. Y

tion 11. The feeding arrangement, however, cannot excite oscillations having an electric field in the direction :1: because the axis of th dipole m is perpendicular or at right angles to the direction :r. ln'the circuit of Fig. 2, use is made of the, fundamental'modes of oscifiation which correspond to, the oscillations excited in the resonator by the feeding dipole m.. By choosingv theproper dimensions, these two fundamental modes of oscillations will have frequencies which difier from each other by a predetermined amount. Generally, it is preferred that these two frequencies be, separated by an amount approxi-, mately two-thirds the-width of the frequency;

band. In a manner discussed in more detailin my copending-application supra, when the electric field is vertical in the z direction, in Fig. 2,

the natural wavelength is given by X=: alaH-b'f when the electric fieldis horizontal in the direction of y, the natural.

- Solong as the above two mathematical re ations a transmission lineTL and dipole m in a manner. corresponding to that of Fig, l for the hollow prism, and the output is obtained by dipole n and transmission line 'I 'L{ also in the manner similar to that of Fig. 1. Dipoles m and n are parallel to each other and make equal angles with the d11- rections :r, u and z. The resonator of Fig. 3 has three natural fundamental frequencies whoseoscillations can be said, in a way, to correspond to the oscillations present in the resonator of Fig. 1, except for the 'more complex electric field distribution. The fundamental modes of oscillationsof the resonator of Fig. 3 are determined by the dimensions of the major and minor axes of the ellipse and also by the height. when the electric field is vertical in the direction 2, the natural fundamental wavelength is determined by the'dimensions of the major and minor axes of the ellipse. When the electric field is horizontal, and principally" in either the direction of the y or the z: axis, the fundamental modes of oscillation are determined by all three dimensions through a very complex relationship..' It is not believed to be necessary to enter into a detailed discussion of this relationship here. In this last case, the di- \mensions can be determined experimentally, that is, by trial and error; or by employing tables of Mathieu functions. Such tables of the Mathieu function of the radial type which may be used in determining'the exact dimension of the elliptic cylinder have been worked out by the Physics Department of ,the Massachusetts Institute of Technology, Cambridge, Mass.

wavelength is given by ner as described above in connection with Fig. 1, it is possible to excite three types of waves corresponding to the three fundamental modes of oscillation, because of the fact that in this cas the only restriction on the wave is that its'magnetic field must lie in a plane parallel to the 11-2 plane; It "should be noted that the substitution of the loops for the dipoles of Fig. 2 provides three fundamental modes of oscillation, whereas with the use of dipoles only two fundamental modes of oscillation are obtained. This statement holds true provided the dimensions a and b of Fig. 2 are different fromeach other. However, if dimensions a and b are equal to each other, then the substitution of loops for dipoles in Fig. 2 could only produce two natural fundamental frequencies of oscillation.

If loops are substituted'for dipoles in the hollow elliptic cylinder resonator of Fig. 3, there will be obtained three natural fundamental fre-, quencies. If loops are substituted for the dipoles of the hollow circular cylindrical tank of Fig. 4, there will result only two natural fundamental frequencies of oscillation, due to the fact that there are only two natural fundamental frequencies of, oscillation in 'a resonator-of such a configuration Since there are only two natural fundamental frequencies of oscillation in the oblate and prolate spheroids of Figs. 6

substitution of loops for dipoles in fll lr s mental modes of oscillation. Asfor th hollow ellipsoid of Fig. 8, the same considerations mentioned above in connection with Fig. 1 apply to the hollow ellipsoid when loops are substituted for'the dipole.

these two n desired the input and output circuits can I consist of concentric lines with the inner conductor entering the interior of the resonator in the manner of a probe for exciting and for driving energy from the resonator.

"Ifhe resonators of the present invention find particular application in the ultra short wavefield and may be used wherever a filter can be used and forsubstantially the same purpose, such as between stages of a receiver and a transmitter. When used as a band pass coupling circuit,

it is preferred thatthe fundamental frequencies .of oscillation caused to exist by exciting onator in the manner described-above be reason- 'ably close to one another, inorder to obtain a smooth band pass characteristic. One application of the presentinventionis to use the resonator-in connection with an inductive output tube wherein an electron stream serves to'excite the resonator. Such an application is generally described in Figs. 11 and 12 of my corresponding application," Serial No. 35am, to which atten I .tion' is directed. The resonator of the presentrf invention may also b used as an input circuit of a frequency-mixer or detector, wherein a pair of relatively close frequencies can be mixed in the resonator and the beat frequency delivered from an electronic tube. This last application is primarily for use in a superheterodyne receiver,

wherein it is desired to obtain an intermediate frequency from a detector.

Whatisclalmedis:

1. A high frequency cavity resonator comprising ahollow closed electrically conducting surface having different principal dimensions, and

means 'for exciting -said resonator in its interior in such manner as to produce oscillations of at least two fundamental mode simultaneously.

, 2. Ahigh frequency-cavity resonator comprisassigns ing a hollow closed electrically conducting surface and 7, the

will, of course, result in only two fundathe res- I having three different principal dimensions, and means for exciting said resonator in its interior in such manner asto produce oscillations of three '5 fundamental modes simultaneously, I

3. A high frequency cavity resonator comprising a hollow closed electrically conducting surface in the form of a rectangular prism having three different principal dimensions, and means for ex- 1 citing said resonator in its interior in such manner as. to produce oscillations .of three fundamenjtal modes simultaneously. v

j 4. A high frequency cavity resonator compris- "fing a hollow closed electrically conducting surla face in the form of an elliptic cylinder tank havini three different principal dimensions, and means for exciting said resonator in its interior in suchmanner as to produce oscillations of three fundamental modes simultaneously.

5. A coupling circuit for passing a band of frequencies comprising a cavity resonator in the form of a hollow closed electrically conducting 4 surface having different predetermined principal dimensions, and means for exciting said res- 6. A coupling circuit for passing a band of fre- I quencies comprising a cavity resonator in the form, of a hollow closed electrically conducting surface having different predetermined princip 1 dimensions, and an exciting circuit in the i;- theinterior of said resonator having an axis maltmg substantially equaltangles with said princiso pal dimensions.

7. A coupling circuit for passing a band of frequencies comprising -a cavity resonator in the fonn of a hollow closed electrically conducting surfacehavingdifferent predetermined principal 40 dimensions, and an exciting circuit in the inte rior of saidresonator having an axis making different desired angles with said' principal dimansions. to produce simultaneously different fundamental modes of oscillation.

8; A coupling circuit for passing a band of frequencies comprising a cavity resonator in the. form or a hollow closed electrically conducting surface having different predetermined principal dimensions, and an exciting circuit in the form of a dipole positioned in the interior of said resonator and having an axis making different desired angles with said principal dimensions to produce smultaneously diiferent fundamental modes of oscillation. 4

9. A coupling circuit for passing a band .of frequencies comprising a cavity resonator in the form of a hollow closed electrically conducting 1 surface having different predetermined principal dimensions, and an exciting circuit in the inte rior of said resonator having an axis making desiredangles with said principal dimensions, and I an output circuit 'of the same form as and par- '70 cillations in said resonator of at least two fundamental'modes simultaneously.

11. A high frequency cavity resonator comprising a hollow closed electrically conducting circular cylinder having only'two different prinzs dimensions, andwreans for exciting said resonator in its interiorin such manner as to produce oscillations in said resonator of at least two fundamental modes simultaneously.

12. A high frequency cavity resonator comprising a hollow closed prismatic electrically conducting surface having different principal dimensions, and means for exciting said resonator in its interior in such manner as to produce oscillations in said resonator of at least two fundamental modes simultaneously.

, 13. A coupling circuit for passing a band of frequencies comprising a cavity resonator in the form of a hollow closed electrically conducting surface having difierent'predetermined principal dimensions, and an exciting circuit in the interior of said resonator having an axis making desired angles with said principal dmensions, and an output circuit parallel to said exciting circuit also located in the interior of said resonator.

14. A high frequency cavity resonator comprisinga hollow closed electrically conducting surface in the form of a rectangular prism having different principal dimensions, and means for exciting said resonator in its interior in such manner as to 1 produce oscillations in said resonator of at least two fundamental modes simultaneously.

15. A high frequency cavity resonator comprising a hollow closed electrically conducting surface having different principal dimensions,

sions.

1'7. A high frequency cavity resonator comprising a hollow closed electrically conducting surface in the form of a rectangular prism having three different principal dimensions, and means for exciting said resonator in its interior in. such manner as to produce oscillations of three fundamental modes simultaneously, said means including an element entering the interior of said prism at onecorner.

'18. A high frequency cavity resonator comprising a hollow closed electrically conducting surface in the form of a rectangular prism having three different principal dimensions, and

a ment.

means for exciting said resonator in its interior in such -manner as to produce oscillations of three fundamental modes simultaneously, said means including an element entering the interior.

of said prism at one comer, and an output circuit comprising an element entering the interior of said prism at the opposite corner.

19. A coupling circuit for passing a band of frequenices comprising a cavity resonator in the form of a hollow closed electrically conducting rectangular prism having different predetermined principal dimensions, and means for exciting said resonator in its interior in such manner that there is caused to exist in said resonator at least,

two fundamental modes of oscillation, said means including an element entering one side of said prism at the center of said side and making equal angles with the edges of said side.

20. A high frequency cavity resonator comprising ahollow closed electrically conducting surface having-three different principal dimensions, and means for exciting said resonator in its interior in such manner as to produce oscillations of three fundamental modes simultaneously, said means including an element entering the interior of said resonator at one comer.

21. A high frequency cavity resonator comprising a hollow closed electrically conducting surface having three different principal dimensions, and means for exciting said resonator in its interior in such manner as to produce oscillations of three fundamental modes simultaneously, said means including an element entering the interior of said resonator at one corner and making equal angles with said three different principal dimensions, and an output circuit comprising an element entering the interior-o1 said resonator at the opposite corner and arranged parallel to said first element.

22. A coupling circuit for passing a band of frequencies comprising a cavity resonator in the form of a hollow closed electrically conducting surface having different predetermined principal dimensions, and means for exciting said resonator in its interior in such manner that there is caused to exist in said resonator at leasttwo fundamental modes of oscillation, said'means including an element entering said resonator at.

a location removed from one corner and making equa1 angles with at least two of said principal dimensions, and an output circuit including an element entering said resonator at another location and arranged parallel to said first'ele- PHILIP s. CARTER. 

